Mark 1:23

And there was in their synagogue a man with an unclean spirit--literally, "in an unclean spirit"--that is, so entirely under demoniacal power that his personality was sunk for the time in that of the spirit. The frequency with which this character of "impurity" is ascribed to evil spirits--some twenty times in the Gospels--is not to be overlooked. and he cried out--as follows:

As soon as Christ began to preach, he began to work miracles for the confirmation of his doctrine; and they were such as intimated the design and tendency of his doctrine, which were to conquer Satan, and cure sick souls. In these verses, we have, I. Christ's casting the devil out of a man that was possessed, in the synagogue at Capernaum. This passage was not related in Matthew, but is afterward in Luk 4:33. There was in the synagogue a man with an unclean spirit, en pneumati akathartō - in an unclean spirit; for the spirit had the man in his possession, and led him captive at his will. So the whole world is said to lie en tō ponerō - in the wicked one. And some have thought it more proper to say, The body is in the soul, because it is governed by it, than the soul in the body. He was in the unclean spirit, as a man is said to be in a fever, or in a frenzy, quite overcome by it. Observe, The devil is here called an unclean spirit, because he has lost all the purity of his nature, because he acts in direct opposition to the Holy Spirit of God, and because with his suggestions he pollutes the spirits of men. This man was in the synagogue; he did not come either to be taught or to be healed, but, as some think, to confront Christ and oppose him, and hinder people from believing on him. Now here we have, 1. The rage which the unclean spirit expressed at Christ; He cried out, as one in an agony, at the presence of Christ, and afraid of being dislodged; thus the devils believe and tremble, have a horror of Christ, but no hope in him, nor reverence for him. We are told what he said, Mar 1:24, where he doth not go about to capitulate with him, or make terms (so far was he from being in league or compact with him), but speaks as one that knew his doom. (1.) He calls him Jesu…

1:23-24 The demon spoke through the man who was possessed by an evil spirit (1:24). Demons frequently cause disease (9:17, 27; Matt 17:15) and self-destructive behavior (Mark 1:26; 5:2-5; 9:17-18, 20-22). Demons know who Jesus is (1:34); they consistently testify that he is the Holy One of God (see 3:11; 5:7). • In saying us, the evil spirit spoke on behalf of all demons. It recognized Jesus’ complete authority and understood that he had come to interfere with and destroy evil.